feat(codec): PR-X12 v1 — λ-RDO mode selection + rANS entropy coder #530

Job	Run time
format/stable	21s
hpc-stream-parallel/rayon	1m 26s
Pass MSRV values to other jobs	2s
clippy/1.95.0	41s
nostd/thumbv6m-none-eabi/stable	26s
tier4-avx512-check	31s
native-backend/stable	7m 52s
nightly-simd-polyfill	0s
docs/nightly	0s
cargo-careful	0s
cross_test/${{ matrix.target }}/${{ matrix.rust }}	0s
miri	0s
tests/beta	10m 48s
tests/stable	11m 31s
tests/1.95.0	10m 38s
blas-msrv	51s
conclusion	-1s
	45m 6s

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